There is an optical disc which enables recording of information signals at a higher recording density than on such optical discs as CD (compact disc) and DVD (digital versatile disk). An optical disc of this type enables recording of information signals at a higher recording density than on CD or DVD by using a light beam having a wavelength of approximately 400 nm, which is shorter than in the case of CD or DVD, for recording or reproducing information signals, and using a numerical aperture (NA) of an objective lens of approximately 0.85, which is larger than in the case of CD or DVD.
For example, the optical disc which enables recording of information signals at a higher recording density than on CD or DVD has a predetermined pattern consisting of lands and grooves formed on one side of a disc substrate, a reflection layer provided for reflecting a light beam onto the predetermined pattern, a signal recording layer made of a phase change material or the like and provided on the reflection layer, and a light-transmitting layer provided on the signal recording layer, and recording or reproduction of information signals is carried out by casting a light beam from the light-transmitting layer side.
Meanwhile, with respect to optical discs, it has been known that a better recording/reproducing characteristic can be realized when recording information signals to a land, which is a protruding part, than when recording information signals to a groove, which is a recess part. Therefore, on the optical disc with a higher recording density of information signals, information signals are recorded to lands by casting a light beam from the optical transmitting layer side, which is opposite to the substrate.
When manufacturing the optical disc with a higher recording density, first, a photoresist is applied on one side of a glass master board and it is exposed and developed to form a recess/protrusion pattern. Nickel-plating or silver-plating is performed thereon to manufacture a disc master. Then, a mother master is manufactured by transferring the recess/protrusion pattern formed on the disc master. This mother master is used as a stamper for forming a predetermined pattern on a disc substrate. Specifically, when manufacturing CD or DVD, a stamper is formed by further transferring the recess/protrusion pattern of the mother, whereas for this optical disc, the mother master can be used as a stamper and a recess part for recording information signal, that is, a land, can be formed on the light-transmitting layer side. On the substrate on which the recess/protrusion pattern is transferred from the stamper as a mother master and thus a predetermined pattern is formed, a reflection layer, a signal recording layer and a light-transmitting layer are sequentially stacked on the side where the predetermined pattern is formed.
Meanwhile, in the case where the ratio of the width of a protruding part to the width of a recess part is set to 1:1 when forming a recess/protrusion pattern on the disc master in order to form a land and a groove on the disc substrate and thus a disc substrate on which the ratio of the width of the land to the width of the groove is 1:1 is formed, if a reflection layer and a signal recording layer are formed on the disc substrate, the ratio of the width of the land to the width of the groove will not be 1:1 and the land will become wider than the groove.
For example, if a predetermined pattern having a recess part with a depth of 20 nm and a land with a width of 0.16 μm, which is half the track pitch of 0.32 μm, is formed on the disc substrate and a signal recording layer is formed thereon, the width of the land will be larger than the width of the groove by approximately 0.02 μm.
To enable reproduction of an information signal-recordable optical disc in a reproducing device for a reproduction-only optical disc having a pit string formed on a disc substrate, the land must be made narrower than the groove to reduce the quantity of reflection of a light beam. This is because the reproducing device for the reproduction-only disc reads out information signals by detecting the reflectivity of a light beam cast onto the optical disc but cannot detect information signals if the reflectivity is too high.
Therefore, in the information signal-recordable optical disc from which reproduction can be carried out by the reproducing device for the reproduction-only optical disc, the width of the land need be less than 50% of the track pitch when the signal recording layer is formed. To satisfy this condition, the width of the recess part for forming the land of the stamper must be less than at least 50%, preferably less than 45% of the track pitch, in consideration of the thickness of the reflection layer and the signal recording layer. However, a cutting machine for exposing a photoresist applied on a glass master board when forming a disc master which is commercially available at present uses a light beam with a minimum wavelength of 257 nm or 266 nm. Even when the diameter of a beam spot is minimized, it is difficult to reduce the width of the track to less than 45% of the track pitch. For example, when the track pitch is 0.32 μm and the depth of the recess part is 20 nm, the width of the recess part is 45% to 50% of the track pitch at best. Moreover, when the width of the recess part is decreased to approximately 45% to 50% of the track pitch, the surface of the stamper will be rough and causes diffuse reflection when reflecting a light beam. Therefore, a good recording/reproducing characteristic cannot be obtained. To realize a good recording/reproducing characteristic, the width of the recess part must be approximately 45% to 65% of the track pitch.